Although we have reached the Healthy People 2010 goal of 80% coverage among U.S. children 19-35 months of age for the primary immunization series, 20% of children still remain under-immunized. In addition, pockets of low immunization coverage and outbreaks of vaccine preventable disease persist. Immunization reminder-recalls have been shown to be successful, but have been limited by the ability to rapidly identify and reach large target populations in a cost-effective manner. Reminder-recalls via text message may offer an innovative opportunity to improve vaccination coverage. Currently, 89% of Americans own a cell phone. Evidence on how to optimally design these systems is not yet available and thus text message immunization reminder-recalls have not been widely implemented. Prior formative research by our team suggests that text messages reminder-recalls may be both effective and well accepted by parents. Preliminary results from our on-going adolescent text messaging project, Text4Health, show great promise with 15.5% (95% CI: 8.7, 22.0) of the 110 parents texted so far returning for a subsequent immunization compared to 0% in the control group (p =0.000) and suggest that text messaging reminders linked to a immunization registry can significantly increase immunization rates. Encouraged by these preliminary results, in the current proposal we plan to extend this intervention by targeting early childhood immunizations, incorporating immunization appointment reminders, and including six family practice and pediatric care settings, expanding beyond those in our adolescent intervention. In this study, we seek to (1) assess parental and provider interest in, preferences for and satisfaction with immunization related text messaging, (2) measure the effectiveness and costs of tailored text message immunization appointment/immunization recall alerts linked to a well-established immunization registry to increase coverage rates and timeliness of the sentinel vaccines of MMR1 and DTaP3. We will target parents of young children from a primarily Latino, medically underserved population. Our methodologies are designed to address key barriers to adoption and scalability and to increase access to low-income populations. This research aims to pave the way to develop a "best practice," allowing any immunization registry, department of health, or large group practice employ this technology to improve immunization coverage. PUBLIC HEALTH RELEVANCE: This proposed research uses text message immunization appointment reminders / immunization recalls for parents of children from medically underserved backgrounds to improve immunization coverage. Successful implementation of text message interventions could transform the current approach to reminder-recalls.